The invention concerns a method and a device for treating liquids such as municipal wastewater, industrial process liquids etc, where air or another gas is added during mixing. In the following will be described the use of the invention for treatment of wastewater.
Wastewater very often contains a big amount of solid pollutants, which must be kept in suspension in order to be possible to transport. In addition the water lacks oxygen, which causes a bad smell and creates a corrosive environment.
In order to solve these problems mixing and aerating devices are arranged for treating the wastewater in pump stations as well as in storing tanks. Examples are shown in Swedish patents 366 013 and 439 592.
The aeration devices may be designed as air nozzles, which are fed by compressors, turbine aerators that create under pressure and suck down air from the atmosphere, ejector nozzles from which a mixture of air and liquid is sprayed down into the water etc.
In order to make the aeration effective, the time during which the air bubbles stay within the water must be long and this means that the air must be fed at a considerable depth. The energy demand is however mainly proportional to the depth and this means that the efficiency is relatively independent of the depth.
As the energy demand is proportional to airflow and the gas exchange to the interface between air and liquid, the efficiency increases when the sizes of the bubbles diminish.
As the aeration requires a lot of energy and constitutes an important part of the energy costs for a purification plant, it is a very interesting subject to make the aeration as effective as possible. A figure that shows how effective the aeration is may be defined as the ratio between the amount of oxygen solved in the water and the amount of used electric energy, (kg O2/kWh). In devices known today, said ratio varies between 1 and 6, where the highest values concern so-called fine bubble systems. The choice of aeration system is normally decided depending on a balancing between costs for investment and the operation performance.
The purpose with the invention is thus to obtain a method and a device which makes possible an effective and energy saving mixing and which simultaneously has a design which requires little space. The invention is stated in the accompanying claims and is described more closely below with reference to the enclosed drawing.